Alongside the development of our industrial society into the advanced information age, the importance of electronic displays which are a medium for displaying and transferring various pieces of information is increasing day by day. Conventionally, cathode ray tubes (CRTs) were widely used therefor but limitations are imposed on the use thereof as a result of the space required to mount them, thus making it difficult to manufacture CRTs of larger sizes, and accordingly CRTs are being replaced by various types of flat panel displays, including liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), and organic electroluminescence (EL) displays. Among such flat panel displays, LCDs in particular are technologically intensive products resulting from a combination of liquid crystal-semiconductor techniques and are advantageous because they are slim and lightweight and consume little power. Therefore, research and development into LCD structures and manufacturing techniques is ongoing. Nowadays, LCDs, which have already been applied to fields such as notebook computers, monitors for desktop computers, portable personal communication devices (including personal digital assistants (PDAs) and mobile phones) and so on, are being manufactured in larger sizes, and thus it is possible to apply LCDs to large-sized televisions such as high-definition (HD) televisions. As a result, LCDs are receiving attention as novel displays able to substitute for CRTs, a term which used to be synonymous with displays.
In LCDs, because the liquid crystals themselves cannot emit light, an additional light source is provided at the back surface thereof so that the intensity of light passing through the liquid crystals in each pixel is controlled to obtain contrast. More specifically, the LCD, serving as a device for adjusting light transmittance using the electrical properties of a liquid crystal material, emits light from a light source lamp mounted to the back surface thereof, and the light thus emitted is passed through various functional prism films or sheets to thus impart the light with uniformity and directionality, after which such controlled light is also passed through a color filter, thereby obtaining red, green, and blue (R, G, B) colors. Furthermore, the LCD uses indirect light emission in which an image is displayed by controlling the contrast of each pixel via electrical methods. As such, a light-emitting device supplying a light source is an important component for determining the quality of the image of the LCD, including luminance and uniformity.
Such a light-emitting device is mainly exemplified by a backlight unit (BLU). Typically, light emitted from a plurality of light sources including a cold cathode fluorescent lamp (CCFL) is sequentially passed through a diffusion plate, a diffusion sheet and a prism sheet, and then reaches a liquid crystal panel. The diffusion sheet plays a role in attaining uniform light intensity over the entire front surface of a screen and simultaneously performs a hiding function so that a device such as the light source mounted under the diffusion sheet is not visible from the front surface. The prism sheet functions to control the path of light so that light rays directed in various directions having passed through the diffusion sheet are transformed within a range of viewing angles θ suitable for enabling the image to be viewed by an observer.
However, in the course of condensing the light, which was emitted from the light source and diffused via the diffusion sheet, by the prism sheet, some light which is incident only in the range of a predetermined angle can be condensed, and other light which is not condensed is lost or re-condensed. As such, a yellow band may be generated at an intersection between the condensed area and the lost area on the prism sheet, and also, a rainbow phenomenon showing waves of rainbow colors may be generated at the lost area and the re-condensed area on the prism sheet, undesirably causing defects.
In order to solve such problems, a protective sheet has been conventionally layered on the prism sheet, but the use thereof has become limited because of the recent trend to reduce the number of sheets to make the display slimmer and simplify the manufacturing process.
Therefore, methods able to prevent the generation of the yellow band or rainbow phenomenon without the use of the protective sheet are required.